Conventional audio speakers compress/heat and rarify/cool air (thus creating sound waves) using mechanical motion of a cone-shaped membrane at the same frequency as the audio frequency. Most cone speakers convert less than 10% of their electrical input energy into audio energy. These speakers are also bulky in part because large enclosures are used to muffle the sound radiating from the backside of the cone (which is out of phase with the front-facing audio waves). Cone speakers also depend on mechanical resonance; a large “woofer” speaker does not efficiently produce high frequency sounds, and a small “tweeter” speaker does not efficiently produce low frequency sounds.
Thermoacoustic (TA) speakers use heating elements to periodically heat air to produce sound waves. TA speakers do not need large enclosures or depend on mechanical resonance like cone speakers. However, TA speakers are terribly inefficient, converting well under 1% of their electrical input into audio waves.
The present invention relates to an improved loudspeaker that includes an array of electrically conductive membrane transducers such as, for example, an array of polyester-metal membrane pumps.
Graphene membranes (also otherwise referred to as “graphene drums”) have been manufactured using a process such as disclosed in Lee et al. Science, 2008, 321, 385-388. PCT Patent Appl. No. PCT/US09/59266 (Pinkerton) (the “PCT US09/59266 application”) described tunneling current switch assemblies having graphene drums (with graphene drums generally having a diameter between about 500 nm and about 1500 nm). PCT Patent Appl. No. PCT/US11/55167 (Pinkerton et al.) and PCT Patent Appl. No. PCT/US11/66497 (Everett et al.) further describe switch assemblies having graphene drums. PCT Patent Appl. No. PCT/US11/23618 (Pinkerton) (the “PCT US11/23618 application”) described a graphene-drum pump and engine system.
FIGS. 1-5 are figures that have been reproduced from FIGS. 27-32 of the Pinkerton '615 application. As set forth in the Pinkerton '615 application:
FIGS. 1A-1E depict an electrically conductive membrane pump/transducer 2700 that utilizes an array of electrically conductive membrane pumps that cause a membrane 2702 to move in phase. FIGS. 1A-1B are cross-sectional views of the pump/transducer that includes electrically conductive members 2701 (in the electrically conductive membrane pumps) and a speaker membrane 2702. Speaker membrane 2702 can be made of a polymer, such as PDMS. Each of the electrically conductive membrane pumps has a membrane 2701 that can deflect toward downward and upwards. Traces 2605 are a metal (like copper, tungsten, or gold). The electrically conductive membrane pumps also have a structural material 2703 (which can be plastic, FR4 (circuit board material), or Kapton® polyimide film (DuPont USA)) and support material 2704 that is an electrical insulator (like oxide, FR4, or Kapton® polyimide film). Support material 2704 can be used to support the pump membrane, support the stator and also serve as the vent structure. Integrating these functions into one element makes device 2700 more compact than it would be with multiple elements performing these functions. All of the non-membrane elements shown in FIG. 1A-1E can be made from printed circuit boards or die stamped sheets, which enhances manufacturability.
Arrows 2706 and 2707 show the direction of fluid flow (i.e., air flow) in the pump/transducer 2700. When the electrically conductive membranes 2701 are deflected downward (as shown in FIG. 1A), air will flow out of the pump/transducer device 2700 (from the electrically conductive membrane pumps) as shown by arrows 2706. Air will also flow from the cavity 2708 into the electrically conductive membrane pumps as shown by arrows 2707 resulting in speaker membrane 2702 moving downward. When the electrically conductive membranes 2701 are deflected upwards (as shown in FIG. 1B), air will flow into the pump/transducer device 2700 (into the electrically conductive membrane pumps) as shown by arrows 2706. Air will also flow into the cavity 2708 from the electrically conductive membrane pumps as shown by arrows 2707 resulting in speaker membrane 2702 moving upward.
FIG. 1C is an overhead view of pump/transducer device 2700. Line 2709 reflects the cross-section that is the viewpoint of cross-sectional views of FIGS. 1A-1B. FIGS. 1D-1E shows the flow of air (arrows 2707 and 2706, respectively) corresponding to the deflection downward of electrically conductive membranes 2701 and speaker membrane 2702 (which is shown in FIG. 1A). The direction of arrows 2707 and 2706 in FIGS. 1D-1E, respectively, are reversed when the deflection is upward (which is shown in FIG. 1B).
The basic operation for pump/transducer 2700 is as follows. A time-varying stator voltage causes the pump membranes 2701 to move and create pressure changes within the speaker chamber 2708. These pressure changes cause the speaker membrane 2702 to move in synch with the pump membranes 2701. This speaker membrane motion produces audible sound.
The ability to stack pumps in a compact way greatly increases the total audio power. Such a pump/transducer stacked system 2800 is shown in FIG. 2.
For the embodiments of the present invention shown in FIGS. 1A-1E and 2, the individual pump membranes 2701 can be smaller or larger than the speaker membrane 2702 and still obtain good performance.
Pump/transducer system 2700 (as well as pump/transducer speaker stacked system 2800) can operate at higher audio frequencies due to axial symmetry (symmetrical with respect to the speaker membrane 2702 center). Each membrane pump is approximately the same distance from the speaker membrane 2702 which minimizes the time delay between pump membrane motion and speaker membrane motion (due to the speed of sound) which in turn allows the pumps to operate at higher pumping/audio frequencies.
It also means that pressure waves from each membrane pump 2701 arrive at the speaker membrane 2702 at about the same time. Otherwise, an audio system could produce pressure waves that are out of synch (due to the difference in distance between each pump and the speaker membrane) and thus these waves can partially cancel (lowering audio power) at certain pumping/audio frequencies.
Pump/transducer system 2700 (as well as pump/transducer speaker stacked system 2800) further exhibit increased audio power. Since all the air enters/exits from the sides of the membrane pump, these pumps can be easily stacked (such as shown in FIG. 2) to significantly increase sound power. Increasing the number of pump stacks (also referred to “pump cards”) from one to four (as shown in FIG. 2) increases audio power by approximately a factor of 16 As can be seen in FIG. 2, the gas within the chamber is sealed by the membrane pump membranes and the speaker membrane. The gas in the sealed chamber can be air or another gas such as sulfur hexafluoride that can withstand higher membrane pump voltages than air.
Audio output is approximately linear with electrical input (resulting in simpler/cheaper electronics/sensors). Another advantage of the design of pump/transducer 2700 is the way the pump membranes 2701 are charged relative to the gates/stators. These are referred to as “stators,” since the term “gate” implies electrical switching. Pump/transducers have a low resistance membrane and the force between the stator and membrane is always attractive. This force also varies as the inverse square of the distance between the pump membrane and stator (and this characteristic can cause the audio output to be nonlinear/distorted with respect to the electrical input). The membrane can also go into “runaway” mode and crash into the stator. Thus, in practice, the amplitude of the membrane in pump/transducer is limited to less than half of its maximum travel (which lowers pumping speed and audio power).
The issues resulting from non-linear operation are solved in the design of pump/transducer 2700 by using a high resistance membrane (preferably a polymer film like Mylar with a small amount of metal vapor deposited on its surface) that is charged by a DC voltage and applying AC voltages to both stators (one stator has an AC voltage that is 180 degrees out of phase with the other stator). A high value resistor (on the order of 108 ohms) may also be placed between the high resistance membrane (on the order of 106 to 1012 ohms per square) and the source of DC voltage to make sure the charge on the membrane remains constant (with respect to audio frequencies).
Because the pump membrane 2701 has relatively high resistance (though low enough to allow it to be charged in several seconds) the electric field between one stator and the other can penetrate the charged membrane. The charges on the membrane interact with the electric field between stator traces to produce a force. Since the electric field from the stators does not vary as the membrane moves (for a given stator voltage) and the total charge on the membrane remains constant, the force on the membrane is constant (for a give stator voltage) at all membrane positions (thus eliminating the runaway condition and allowing the membrane to move within its full range of travel). The electrostatic force (which is approximately independent of pump membrane position) on the membrane increases linearly with the electric field of the stators (which in turn is proportional to the voltage applied to the stators) and as a result the pump membrane motion (and also the speaker membrane 2702 that is being driven by the pumping action of the pump membrane 2701) is linear with stator input voltage. This linear link between stator voltage and pump membrane motion (and thus speaker membrane motion) enables a music voltage signal to be routed directly into the stators to produce high quality (low distortion) music.
FIG. 3 depicts an electrically conductive membrane pump/transducer 3000 that is similar to the pump/transducers 2700 and 2900, in that it utilizes an array of electrically conductive membrane pumps. Pump/transducer 3000 does not utilize a speaker membrane (such as in pump/transducer 2700) or a structure in place of the speaker membrane (such as in pump/transducer 2900). Pump/transducer 3000 produces substantial sound even without a speaker membrane. Applicant believes the reason that there is still good sound power is that the membrane pumps are compressing the air as it makes its way out of the inner vents (increasing the pressure of an time-varying air stream increases its audio power). Arrows 3001 show the flow of air through the inner vents. The pump/transducer 3000 has a chamber that receives airflow 3001 and this airflow exhausts out the chamber by passing through the open area (the chamber exhaust area) at the top of the chamber. In order to produce substantial sound the total area of the membrane pumps must be at least 10 times larger than the chamber exhaust area.
FIG. 3 also shows an alternate vent configuration that has holes 3003 in the stators that allow air to flow to separate vent layers. The cross-sectional airflow area of the vents (through which the air flow is shown by arrows 3001) is much smaller than the pump membrane area (so that the air is compressed). FIG. 3 also shows how a simple housing 3004 can direct the desired sound 3005 toward the listener (up as shown in FIG. 3) and the undesired out of phase sound away from the listener (down as shown in FIG. 3). The desired sound 3005 is in the low sub-woofer range to mid-range (20 Hz to about 3000 Hz).
FIG. 4 depicts an electrically conductive membrane pump/transducer 3100 that is the pump/transducer 3000 that also includes an electrostatic speaker 3101 (which operates as a “tweeter”). An electrostatic speaker is a speaker design in which sound is generated by the force exerted on a membrane suspended in an electrostatic field. The desired sound 3102 from the electrostatic speakers 3101 is in a frequency in the range of around 2 to 20 KHz (generally considered to be the upper limit of human hearing). Accordingly, pump/transducer 3100 is a combination system that includes a low/mid-range speaker and a tweeter speaker.
FIG. 5 depicts an electrically conductive membrane pump/transducer 3200 that is the pump/transducer 3100 that further includes the speaker membrane 3202 (such as in pump/transducer 2700).